Optical bodies containing cholesteric liquid crystal material and methods of manufacture

ABSTRACT

An optical body includes a substrate and a cholesteric liquid crystal layer disposed on the substrate. The cholesteric liquid crystal layer has a non-uniform pitch along a thickness direction of the layer and comprises a crosslinked polymer material that substantially fixes the cholesteric liquid crystal layer. The crosslinking hinders diffusion of cholesteric liquid crystal material within the cholesteric liquid crystal layer. In other methods of making an optical body, a reservoir of chiral material is provided during the process over a first cholesteric liquid crystal layer to diffuse into the layer and provide a non-uniform pitch. Alternatively, two coating compositions can be disposed on a substrate where the material of the first coating composition is not substantially soluble in the second coating composition.

FIELD OF THE INVENTION

The present invention relates to optical bodies containing cholestericliquid crystals. The present invention also relates to reflectiveoptical polarizers formed by coating of two or more layers ofcholesteric liquid crystals or cholesteric liquid crystal precursors.

BACKGROUND OF THE INVENTION

Optical devices, such as polarizers and mirrors, are useful in a varietyof applications including liquid crystal displays (LCD's). Liquidcrystal displays fall broadly into two categories: backlit (e.g.,transmissive) displays, where light is provided from behind the displaypanel, and frontlit (e.g., reflective) displays, where light is providedfrom the front of the display (e.g., ambient light). These two displaymodes can be combined to form transflective displays that can bebacklit, for example, under dim light conditions or read under brightambient light.

Conventional backlit LCDs typically use absorbing polarizers and canhave less than 10% light transmission. Conventional reflective LCDs arealso based on absorbing polarizers and typically have less than 25%reflectivity. The low transmission or reflectance of these displaysreduces display contrast and brightness and can require high powerconsumption.

Reflective polarizers have been developed for use in displays and otherapplications. Reflective polarizers preferentially transmit light of onepolarization and preferentially reflect light having an orthogonalpolarization. It is preferred that reflective polarizers transmit andreflect light without absorbing relatively large amounts of the light.Preferably, the reflective polarizer has no more than 10% absorption forthe transmission polarization. Most LCD's operate over a broad range ofwavelengths and, as a consequence, the reflective polarizer musttypically operate over that broad wavelength range, as well.

In backlit displays, the reflective polarizer can be used to increasethe efficiency of light utilization by reflecting the polarization ofthe light not transmitted by the polarizer back into the backlight. Thebacklight converts the polarization state of the recycled light fortransmission through the reflective polarizer. This light recycling canincrease total display brightness. In reflective and transflectivedisplays, the reflective polarizer has lower absorptivity and color thanmost absorbing polarizers for the pass polarization of light, and canincrease brightness of the display by up to 50% or more. Characteristicsof reflective polarizers that are important to at least someapplications include, for example, the thickness of the polarizer, theuniformity of reflection over a wavelength range, and the relativeamount of light reflected over the wavelength range of interest.

SUMMARY OF THE INVENTION

Generally, the present invention relates to optical bodies containingcholesteric liquid crystals and their manufacture, as well as the use ofcholesteric liquid crystals in optical devices, such as reflectivepolarizers.

One embodiment of the invention is a method of making an optical body.First, a cholesteric liquid crystal polymer layer is formed on asubstrate using a first coating composition. Next, a second coatingcomposition comprising at least one chiral monomer material selectedfrom reactive chiral monomers is coated on the first layer. Next, aportion of the chiral monomer material is allowed to diffuse into aportion of the first cholesteric liquid crystal polymer layer which isadjacent to the second coating composition. Finally, the chiral monomermaterial is cured and one or more cholesteric liquid crystal layer(s)are generated from the first cholesteric liquid crystal polymer layerand the second coating composition. The cholesteric liquid crystallayer(s) generated have a non-uniform pitch.

Another embodiment of the invention is another method of making anoptical body. First, a first layer is formed on a substrate using afirst coating composition which comprises at least one cholestericliquid crystal material selected from cholesteric liquid crystalcompounds and cholesteric liquid crystal monomers. Next, a secondcoating composition comprising at least one chiral monomer materialselected from reactive chiral monomers is coated on the first layer.Next, a portion of the chiral monomer material is allowed to diffuseinto a portion of the first layer adjacent to the second coatingcomposition. Finally, the chiral monomer material is crosslinked withthe first layer to generate and fix one or more cholesteric liquidcrystal layer(s). The cholesteric liquid crystal layer(s) have anon-uniform pitch and the crosslinking substantially hinders furtherdiffusion of any remaining chiral monomer material.

Another embodiment of the invention is another method of making anoptical body. First, a layer is formed on a substrate using a firstcoating composition. Next, a second coating composition is coated ontothe first layer. The first and second coating compositions are differentand each compositions comprise at least one chiral material selectedfrom chiral compounds. The second coating composition further comprisesa solvent, where the first layer is substantially insoluble in thesolvent of the second coating composition. Next, a portion of the secondcoating composition is allowed to diffuse into a portion of the firstlayer, which is adjacent to the second coating composition. Afterdiffusing, one or more cholesteric liquid crystal layer(s) are formedfrom the second coating composition and the first layer.

Another embodiment of the invention is an optical body which comprises asubstrate and a cholesteric liquid crystal layer disposed on thesubstrate. The cholesteric liquid crystal layer has a non-uniform pitchalong a thickness direction of the layer and comprises a crosslinkedpolymer material that substantially fixes the cholesteric liquid crystallayer. The crosslinking hinders diffusion of cholesteric liquid crystalmaterial within the cholesteric liquid crystal layer.

Another embodiment of the invention is an optical display whichcomprises a display medium and a reflective polarizer. The reflectivepolarizer comprises a substrate and a cholesteric liquid crystal layerdisposed on the substrate. The cholesteric liquid crystal layer has anon-uniform pitch along a thickness direction of the layer and comprisesa crosslinked polymer material that substantially fixes the cholestericliquid crystal layer. The crosslinking hinders diffusion of cholestericliquid crystal material within the cholesteric liquid crystal layer.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic representation of one embodiment of a method andapparatus for sequentially coating two or more cholesteric liquidcrystal compositions on a substrate, according to the invention;

FIG. 2 is a schematic representation of a cross-section of a firstcoating composition on a substrate, according to the invention;

FIG. 3 is a schematic representation of a cross-section of first andsecond coating compositions on a substrate, according to the invention;

FIG. 4 is a schematic representation of a cross-section of the first andsecond coating compositions on a substrate with a region of diffusion,according to the invention;

FIG. 5 is a schematic illustration of one embodiment of a liquid crystaldisplay, according to the invention;

FIG. 6 is a schematic illustration of another embodiment of a liquidcrystal display, according to the invention;

FIG. 7 is a light transmission spectrum of an optical body formedaccording to Example 1;

FIG. 8 is a light transmission spectrum of an optical body formedaccording to Example 2;

FIG. 9 is a light transmission spectrum of an optical body formedaccording to Example 3; and

FIG. 10 is a light transmission spectrum of an optical body formedaccording to Example 4.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is believed to be applicable to optical bodies(such as optical films) and their manufacture, as well as the use of theoptical bodies in optical devices, such as reflective polarizers andoptical displays (e.g., liquid crystal displays). The present inventionis also directed to optical bodies containing cholesteric liquidcrystals. While the present invention is not so limited, an appreciationof various aspects of the invention will be gained through a discussionof the examples provided below.

The term “polymer” will be understood to include polymers, copolymers(e.g., polymers formed using two or more different monomers), andcombinations thereof, as well as polymers or copolymers that can beformed in a miscible blend by, for example, coextrusion or reaction,including transesterification. Both block and random copolymers areincluded, unless indicated otherwise.

The term “polymeric material” will be understood to include polymers, asdefined above, and other organic or inorganic additives, such as, forexample, antioxidants, stabilizers, antiozonants, plasticizers, dyes,and pigments.

The term “cholesteric liquid crystal compound” refers to compounds(including polymers) that can form a cholesteric liquid crystal phase.

The term “chiral material” refers to chiral compounds, including chiralliquid crystal compounds and chiral non-liquid crystal compounds, thatcan form or induce a cholesteric liquid crystal phase in combinationwith other liquid crystal material.

All index of refraction values are reported for 632.8 nm light unlessotherwise indicated.

The term “polarization” refers to plane polarization, circularpolarization, elliptical polarization, or any other nonrandompolarization state in which the electric vector of the beam of lightdoes not change direction randomly, but either maintains a constantorientation or varies in a systematic manner. In plane polarization, theelectric vector remains in a single plane, while in circular orelliptical polarization, the electric vector of the beam of lightrotates in a systematic manner.

Reflective polarizers preferentially transmit light of one polarizationand reflect the remaining light. In the case of reflective planepolarizers, light polarized in one plane is preferentially transmitted,while light polarized in the orthogonal plane is preferentiallyreflected. In the case of circular reflective polarizers, lightcircularly polarized in one sense, which may be the clockwise orcounterclockwise sense (also referred to as right or left circularpolarization), is preferentially transmitted and light polarized in theopposite sense is preferentially reflected. One type of circularpolarizer includes cholesteric liquid crystal polarizers.

Cholesteric Liquid Crystal Compounds

Cholesteric liquid crystal materials typically include molecular unitsthat are chiral in nature (e.g., molecules that do not possess a mirrorplane) and molecular units that are mesogenic in nature (e.g., moleculesthat exhibit liquid crystal phases) and can be polymers. Cholestericliquid crystal compositions include compounds having a cholestericliquid crystal phase in which the director (the unit vector thatspecifies the direction of average local molecular alignment) of theliquid crystal rotates in a helical fashion along the dimensionperpendicular to the director. Cholesteric liquid crystal compositionsare also referred to as chiral nematic liquid crystal compositions. Thepitch of the cholesteric liquid crystal compound is the distance (in adirection perpendicular to the director) that it takes for the directorto rotate through 360°. This distance is typically 100 nm or more.

The pitch of a cholesteric liquid crystal material can typically bealtered by mixing or otherwise combining (e.g., by copolymerization) achiral compound with a nematic liquid crystal compound. The cholestericphase can also be induced by a chiral non-liquid crystal material. Thepitch depends on the relative ratios by weight of the chiral compoundand the nematic liquid crystal compound. The helical twist of thedirector results in a spatially periodic variation in the dielectrictensor of the material, which in turn gives rise to the wavelengthselective reflection of light. For light propagating along the helicalaxis, Bragg reflection generally occurs when the wavelength, λ, is inthe following rangen _(o) p<λ<n _(e) pwhere p is the pitch and n_(o) and n_(e) are the principal refractiveindices of the cholesteric liquid crystal material. For example, thepitch can be selected such that the Bragg reflection is peaked in thevisible, ultraviolet, or infrared wavelength regimes of light.

Cholesteric liquid crystal compounds, including cholesteric liquidcrystal polymers, are generally known and typically any of thesematerials can be used to make optical bodies. Examples of suitablecholesteric liquid crystal polymers are described in U.S. Pat. Nos.4,293,435 and 5,332,522, 5,886,242, 5,847,068, 5,780,629, 5,744,057 allof which are incorporated herein by reference. Other cholesteric liquidcrystal compounds can also be used. Typically, a cholesteric liquidcrystal compound is selected for a particular application or opticalbody based on one or more factors including, for example, refractiveindices, pitch, processability, clarity, color, low absorption in thewavelength of interest, compatibility with other components (e.g., anematic liquid crystal compound), ease of manufacture, availability ofthe liquid crystal compound or monomers to form a liquid crystalpolymer, rheology, method and requirements of curing, ease of solventremoval, physical and chemical properties (for example, flexibility,tensile strength, solvent resistance, scratch resistance, and phasetransition temperature), and ease of purification.

Cholesteric liquid crystal polymers are typically formed using chiral(or a mixture of chiral and achiral) molecules (including monomers) thatcan include a mesogenic group (e.g., a rigid group that typically has arod-like structure to facilitate formation of a cholesteric liquidcrystal phase). Mesogenic groups include, for example, para-substitutedcyclic groups (e.g., para-substituted benzene rings). The mesogenicgroups are optionally bonded to a polymer backbone through a spacer. Thespacer can contain functional groups having, for example, benzene,pyridine, pyrimidine, alkyne, ester, alkylene, alkene, ether, thioether,thioester, and amide functionalities. The length or type of spacer canbe altered to provide different solubilities in solvents.

Suitable cholesteric liquid crystal polymers include polymers having achiral or achiral polyester, polycarbonate, polyamide, polyacrylate,polymethacrylate, polysiloxane, or polyesterimide backbone that includemesogenic groups optionally separated by rigid or flexible comonomers.Other suitable cholesteric liquid crystal polymers have a polymerbackbone (for example, a polyacrylate, polymethacrylate, polysiloxane,polyolefin, or polymalonate backbone) with chiral and achiral mesogenicside-chain groups. The side-chain groups are optionally separated fromthe backbone by a spacer, such as an alkylene or alkylene oxide spacer,to provide flexibility.

Typically, to form a cholesteric liquid crystal layer, a cholestericliquid crystal composition is coated or otherwise disposed onto asurface. The cholesteric liquid crystal composition includes a chiralcomponent containing at least one (i) chiral compound, (ii) chiralmonomer that can be used (e.g., polymerized or crosslinked) to form acholesteric liquid crystal polymer, or (iii) a combination thereof. Thecholesteric liquid crystal composition can also include a non-chiralcomponent that contains at least one (i) nematic liquid crystalcompound, (ii) nematic liquid crystal monomer that can be used to form anematic liquid crystal polymer, or (iii) a combination thereof. Thenematic liquid crystal compound(s) or nematic liquid crystal monomerscan be used to modify the pitch of the cholesteric liquid crystalcomposition. The cholesteric liquid crystal composition can also includeone or more additives, such as, for example, curing agents,crosslinkers, antiozonants, antioxidants, plasticizers, stabilizers, andultraviolet, infrared, or visible light-absorbing dyes and pigments.

Cholesteric liquid crystal compositions can also be formed using two ormore different types of any of the following: chiral compounds, achiralcompounds, cholesteric liquid crystals, cholesteric liquid crystalmonomers, nematic liquid crystals, nematic liquid crystal monomers,latent nematic or chiral nematic materials (in which the latent materialexhibits the liquid crystal mesophase in combination with othermaterials), or combinations thereof. The particular ratio(s) by weightof materials in the cholesteric liquid crystal composition willtypically determine, at least in part, the pitch of the cholestericliquid crystal layer.

The cholesteric liquid crystal composition is generally part of acoating composition that also typically includes a solvent. The term“solvent”, as used herein, also refers to dispersants and combinationsof two or more solvents and dispersants. In some instances, one or moreof the liquid crystals, liquid crystal monomers, processing additives,or any other component of the cholesteric liquid crystal compositionalso acts as a solvent. The solvent can be substantially eliminated fromthe coating composition by, for example, drying the composition toevaporate the solvent or reacting a portion of the solvent (e.g.,reacting a solvating liquid crystal monomer to form a liquid crystalpolymer).

After coating, the cholesteric liquid crystal composition is convertedinto a liquid crystal layer. This conversion can be accomplished by avariety of techniques including evaporation of a solvent; crosslinkingthe cholesteric liquid crystal composition; or curing (e.g.,polymerizing).the cholesteric liquid crystal composition using, forexample, heat, radiation (e.g., actinic radiation), light (e.g.,ultraviolet, visible, or infrared light), an electron beam, or acombination of these or like techniques.

Optionally, initiators can be included within the cholesteric liquidcrystal composition to initiate polymerization or crosslinking ofmonomeric components of the composition. Examples of suitable initiatorsinclude those that can generate free radicals to initiate and propagatepolymerization or crosslinking. Free radical generators can also bechosen according to stability or half-life. Preferably the free radicalinitiator does not generate any additional color in the cholestericliquid crystal layer by absorption or other means. Examples of suitablefree radical initiators include thermal free radical initiators andphotoinitiators. Thermal free radical initiators include, for exampleperoxides, persulfates, or azonitrile compounds. These free radicalinitiators generate free radicals upon thermal decomposition.

Photoinitiators can be activated by electromagnetic radiation orparticle irradiation. Examples of suitable photoinitiators include,onium salt photoinitiators, organometallic photoinitiators, metal saltcationic photoinitiators, photodecomposable organosilanes, latentsulphonic acids, phosphine oxides, cyclohexyl phenyl ketones, aminesubstituted acetophenones, and benzophenones. Generally, ultraviolet(UV) irradiation is used to activate the photoinitiator, although otherlight sources can be used. Photoinitiators can be chosen based on theabsorption of particular wavelengths of light.

The cholesteric liquid crystal phase can be achieved using conventionaltreatments. For example, a method of developing a cholesteric liquidcrystal phase includes depositing the cholesteric liquid crystalcomposition on an oriented substrate. The substrate can be orientedusing, for example, drawing techniques or rubbing with a rayon or othercloth. Photoalignment layers are described in U.S. Pat. Nos. 4,974,941,5,032,009, 5,389,698, 5,602,661, 5,838,407, and 5,958,293. Afterdeposition, the cholesteric liquid crystal composition is heated abovethe glass transition temperature of the composition to the liquidcrystal phase. The composition is then cooled below the glass transitiontemperature and the composition remains in the liquid crystal phase.

Cholesteric Liquid Crystal Optical Bodies

Cholesteric liquid crystal compositions can be formed into a layer thatsubstantially reflects light having one circular polarization (e.g.,left or right circularly polarized light) and substantially transmitslight having the other circular polarization (e.g., right or leftcircularly polarized light) over a particular bandwidth of lightwavelengths. This characterization describes the reflection ortransmission of light directed at normal incidence to the director ofthe cholesteric liquid crystal material. Light that is directed at otherangles will typically be elliptically polarized by the cholestericliquid crystal material and the Bragg reflection peak is typicallyblue-shifted from its on-axis wavelength. Cholesteric liquid crystalmaterials are generally characterized with respect to normally incidentlight, as done below, however, it will be recognized that the responseof these materials can be determined for non-normally incident lightusing known techniques.

The cholesteric liquid crystal layer can be used alone or in combinationwith other layers or devices to form an optical body, such as, forexample, a reflective polarizer. Cholesteric liquid crystal polarizersare used in one type of reflective polarizer. The pitch of a cholestericliquid crystal polarizer is similar to the optical layer thickness ofmultilayer reflective polarizers. Pitch and optical layer thicknessdetermine the center wavelength of the cholesteric liquid crystalpolarizers and multilayer reflective polarizers, respectively. Therotating director of the cholesteric liquid crystal polarizer formsoptical repeat units similar to the use of multiple layers having thesame optical layer thickness in multilayer reflective polarizers.

The center wavelength, λ₀, and the spectral bandwidth, Δλ, of the lightreflected by the cholesteric liquid crystal layer depend on the pitch,p, of the cholesteric liquid crystal. The center wavelength, λ₀, isapproximated by:λ₀=0.5(n _(o) +n _(e))pwhere n_(o) and n_(e) are the refractive indices of the cholestericliquid crystal for light polarized parallel to the director of theliquid crystal (n_(e)) and for light polarized perpendicular to thedirector of the liquid crystal (n_(o)). The spectral bandwidth, Δλ, isapproximated by:Δλ=2λ₀(n _(e) −n _(o))/(n _(e) +n _(o))=p(n _(e) −n _(o)).

Cholesteric liquid crystal polarizers have been previously formed bylaminating or otherwise stacking two already-formed cholesteric liquidcrystal layers, each disposed on an individual substrate, with differentpitches (e.g., layers having different compositions, for example,different ratios by weight of chiral and nematic liquid crystalcompounds or monomers). These two layers are heated to diffuse liquidcrystal material between the layers. The diffusion of material betweenthe two layers typically results in the pitch of the layers varying overa range between the individual pitches of the two layers.

This method, however, requires a substantial number of processing stepsincluding separately forming each layer (e.g., individually drying orcuring each layer), stacking (e.g., laminating) the layers, and thenheating the layers to cause diffusion of liquid crystal material betweenthe two layers. This also requires substantial processing time,particularly, in view of the time required for diffusion between the twopreviously formed liquid crystal layers which are typically polymeric innature.

New Methods of Making Cholesteric Liquid Crystal Optical Bodies

New techniques for making cholesteric liquid crystal optical bodies havebeen developed. These techniques include one or more of the followingfeatures: (i) solvent and material selection to facilitate sequentialcoating, (ii) use of a reservoir of chiral cholesteric liquid crystalmaterial, and (iii) crosslinking to “fix” the cholesteric liquid crystallayer(s). Each of these features is discussed individually; however, itwill be recognized that these features can be used in any combination.

One new method of forming cholesteric liquid crystal optical bodiesincludes sequentially coating at least two different coatingcompositions onto a substrate, each of the coating compositionsincluding a different cholesteric liquid crystal composition. Aftercoating, material from the second coating composition is diffused intothe first coating composition, followed by final conversion into thecholesteric liquid crystal layer(s). The two different liquid crystalcompositions each include a solvent; the two solvents being different.In one embodiment, the cholesteric liquid crystal composition of thefirst coating composition is not soluble in the solvent used with thesecond coating composition. The second coating composition includeschiral monomers (e.g., chiral monomers or cholesteric liquid crystalmonomers) that can be polymerized to form cholesteric liquid crystals.The first coating composition can include a polymeric or monomericcholesteric liquid crystal composition.

In the method, the first coating composition is disposed on thesubstrate using any technique, such as any coating technique. The secondcoating composition is then disposed on the first coating composition.Because the cholesteric liquid crystal composition of the first coatingcomposition is not substantially soluble in the solvent of the secondcoating composition, disruption to the first coating composition by thesolvent is avoided or reduced. After disposing the second coatingcomposition on the first coating composition, the second coatingcomposition may diffuse into the first coating composition. Optionally,the first coating composition is polymerized, if the compositioncontains polymerizable material, prior to diffusion. Followingdiffusion, the solvent of the second coating composition is removed andthe compositions are converted into a liquid crystal layer(s).

In another embodiment of this method, the first coating compositionincludes polymerizable monomers. After disposing the first coatingcomposition on the substrate, the first coating composition is partiallyor fully polymerized. The second coating composition is disposed on thepolymerized first coating composition and the method proceeds asdescribed above. In this embodiment, the polymerized first coatingcomposition should be substantially insoluble in the solvent of thesecond coating composition. There is no requirement with respect to thesolubility of the unpolymerized first coating composition in the solventof the second coating composition.

In at least some instances, the polymerization of the first coatingcomposition can result in a molecular weight gradient along thethickness direction of the layer formed by the composition. Generally,the molecular weight gradient is prepared so that the highest molecularweight material is near the substrate and lower molecular weightmaterial is near the surface upon which the second coating compositionis to be disposed. This gradient can facilitate control of diffusion ofthe second coating composition into the polymerized first coatingcomposition. Diffusion is typically slower through higher molecularweight material. Other gradients, such as low molecular weight polymernear the substrate and high molecular weight polymer near the othersurface, can be used.

A variety of techniques can be used to make the molecular weightgradient. One method includes only partially curing (e.g., polymerizing)the first coating composition. Partial curing can be accomplished byreducing the length or intensity of heating, radiation, light exposure,or a combination thereof. Generally, the source of curing radiation orheat for this technique is placed closest to the surface where thehighest molecular weight material is desired. In some instances, thefirst coating composition includes a material that absorbs the curingradiation to reduce the amount of curing radiation transmitted along thethickness direction. The partial curing can result in the formation of apolymer layer that contains a gradient of molecular weight.

Another method of forming a molecular weight gradient includes curingthe first coating composition in an atmosphere containing oxygen (e.g.,air or another oxygen gas mixture) or another polymerization terminationcomponent. Cholesteric liquid crystal material that is proximal to thesurface of the layer in contact with oxygen (e.g., in air) will notpolymerize as readily as the surface that is not in contact with oxygen(e.g., the substrate surface). This is particularly true when thepolymerization occurs via a free radical process. Oxygen is known toreduce the amount of available free radicals, thereby terminating orotherwise inhibiting polymerization reactions. This oxygen inhibition ofpolymerization can result in a gradient of polymerized materialestablished along the thickness direction of the layer.

To avoid the effect of oxygen inhibition, polymerization can also takeplace under conditions where minimal or no oxygen is present, forexample, under a nitrogen atmosphere. The presence of nitrogen duringpolymerization will typically not substantially inhibit thepolymerization at the interface. In this instance, the molecular weightis typically uniform and high along the thickness of the layer.

Another method that can be used individually or in combination with themethods discussed herein includes utilizing the second coatingcomposition as a reservoir of chiral material (e.g., chiral compounds,cholesteric liquid crystal compounds or cholesteric liquid crystalmonomers). In one embodiment of this method, the second coatingcomposition can be chosen to produce a cholesteric liquid crystalmaterial having a pitch that places its center reflection wavelengthoutside the desired range of wavelengths to be reflected by the opticalbody. For example, for visible light reflective polarizers, the secondcoating composition can be selected to produce a cholesteric liquidcrystal material having a pitch that provides for reflection of infraredor ultraviolet light. In addition, the second coating composition ispreferably selected to permit faster diffusion of the chiral materialthan diffusion of non-chiral materials, such as the nematic liquidcrystal compounds or monomers. One example is a selection of chiralmaterials that are more soluble than the non-chiral materials in thepreviously deposited layer.

Diffusion of the chiral components of the second coating compositioninto the previously deposited layer will change the ratio of chiral tonon-chiral components in the second coating composition. This changesthe pitch of a cholesteric liquid crystal material that can be formedfrom the second coating composition. However, because the centerreflection wavelength is outside the desired wavelength range ofreflection, the change in pitch does not substantially affect thedesired optical properties to be obtained by that portion of thestructure formed using the second coating composition. In an alternativeembodiment, the second coating composition is not a cholesteric liquidcrystal composition, but only contains the chiral component(s) necessaryto alter the pitch of a portion of the layer formed using the firstcoating composition. The concentration, or the percentage of chiralmaterial in the second coating composition can be sufficient so thatdiffusion the chiral material into the first layer does not reduce thequantity of chiral material necessary to give the desired opticalproperties of the optical body. As another option, the second coatingcomposition can include diffusible achiral material that can alter thepitch of a cholesteric liquid crystal material formed by the firstcoating composition. Further examples will be discussed where chiralmaterials are used for diffusion, however, it will be recognized thatthe same structures and objective can be achieved using achiral materialin place of the chiral materials.

In another method, the second coating composition includes reactivemonomer material that can crosslink, in addition to polymerize.Preferably, this reactive monomer material is a reactive chiral monomerand, in some embodiments is a cholesteric liquid crystal compound, aprecursor for a cholesteric liquid crystal polymer, or a chiralcompound. For example, the reactive monomer material can be adi(meth)acrylate, a diepoxide, a divinyl, or a diallyl ether. When thisreactive monomer material diffuses into the previously formed layer, thereactive monomer material can crosslink within that layer as well aswithin the second coating composition. This “fixes” the cholestericliquid crystal layer(s) and prevents or substantially reduces anyfurther diffusion of material within the layer(s).

This method and configuration has advantages over previous techniques,in which there was heat-induced diffusion to mix portions of cholestericliquid crystal polymer layers. In these techniques, the resultingproduct would continue to experience diffusion between layers ofdifferent composition over time, particularly when the product wasutilized in an application with substantial heat production, such asmany display applications. This continued diffusion resulted in changesin the optical properties of the product over time.

In contrast, the technique disclosed herein for crosslinking thecholesteric liquid crystal layer(s) provides a method for substantiallyreducing or preventing further diffusion after crosslinking byincreasing molecular weight and reducing the availability of monomermaterials for diffusion. Thus, the optical properties of the resultingoptical body can be substantially stable over time and can be used toproduce a more reliable product with a longer lifetime.

The methods described above can be performed using a variety oftechniques and equipment. FIG. 1 illustrates an example of a suitablemethod and device for accomplishing the sequential coating of the two ormore coating compositions onto a substrate. A sequential coatingapparatus 100 includes a carrier (e.g., a conveyor belt or a slidingplatform) that conveys the substrate 200 past a first coating dispenser104. Alternatively, the substrate 200 can be a continuous web that ispulled through the apparatus 100 through use of drive rolls. The use ofdrive rolls, or a similar mechanism, for moving the substrate 200 andone or more coating layers can eliminate the necessity for a carrier102, located underneath the substrate 200. The first coating composition202 is dispensed through a first coating head 106 and onto the substrate200. Any coating technique can be used including, for example, knifecoating, bar coating, slot coating, gravure coating, roll coating, spraycoating, or curtain coating. In one embodiment, the first coatingcomposition 202 includes a solvent and a polymeric liquid crystalmaterial or monomers that can be partially or fully polymerized beforecoating or after coating to form a polymeric liquid crystal material.

The first coating composition 202 and substrate 200 can optionally passthrough a drying oven 108 to remove solvent. Also, the first coatingcomposition 202 and substrate 200 can be passed through a curing station110 containing, for example, a heat or light source to polymerize(partially or fully) the first coating composition, if the compositioncontains polymerizable components and it is desired to polymerize thosecomponents at this stage of the process. The curing station 110 can beplaced at one or more various positions relative to the position of thesubstrate 200 and first coating composition 202.

A second coating dispenser 112 then dispenses a second coatingcomposition 204 through a second coating head 114 onto the first coatingcomposition 202. Again, any coating technique can be used. Preferably,as discussed above, the layer previously formed from the first coatingcomposition is not substantially soluble in the solvent of the secondcoating composition. The second coating composition 204 includes asolvent and chiral material (e.g., cholesteric liquid crystal monomer orother chiral compounds or a mixture of nematic (e.g., non-chiral) andchiral liquid crystal monomer or other chiral compounds). At least someof the chiral material of the second coating composition 204 is at leastpartially soluble in the solvent of the first coating composition.Preferably, the second coating composition contains sufficient chiralmaterial to produce a layer that reflects light outside the desiredwavelength range for reflection by the optical body. In this manner, thesecond coating composition can act as a reservoir for providing chiralmaterial to the layer formed using the first coating composition withoutdecreasing the ability of the subsequently produced optical body tocover the desired wavelength range. Any change in the pitch ofcholesteric liquid crystal formed from the second coating composition(if the second coating composition can form a cholesteric liquidcrystal) will preferably only be observable outside the desiredwavelength range.

When the first and second coating compositions are in contact with oneanother, the diffusion of the chiral material of the second coatingcomposition into the first coating composition 202 can take place. Thisdiffusion of the chiral material can result in cholesteric liquidcrystals that have an intermediate pitch. The intermediate pitch isbetween the pitches of the cholesteric liquid crystal layers that can beformed from the first and second coating compositions alone.

The rate of diffusion depends upon a variety of factors including, forexample, the specific materials used in each composition, thepercentages of materials in these compositions, the molecular weight ofthe materials, the temperature of the compositions, the viscosity of thecompositions, and the degree of polymerization of each composition. Adesired diffusion rate can be obtained by controlling one or more ofthese variables, for example, by choice of materials, temperature,viscosity, polymer molecular weight, or a combination of thesevariables. The first and second coating compositions are optionallyplaced in an oven 116 or other heating unit to increase the diffusionrate of the chiral material of the second coating composition into thelayer formed using the first coating composition. This oven can also beused to partially or fully remove the solvents from the first and secondcoating compositions, if desired.

After a desired degree of diffusion is achieved, the first and secondcoating compositions are fully cured using a curing station 118including, for example, a light or heat source. In one embodiment, asdescribed above, the second coating composition includes a material thatcan diffuse into the layer formed using the first coating compositionand crosslink the materials within that layer and within the secondcoating composition.

The speed of the substrate 200 and the flow rate of the coatingcompositions are controlled to provide the desired thickness of each ofthe compositions 202 and 204. the speed of the substrate 200 can also becontrolled to change the duration of treatment by the ovens 108/116 orcuring stations 110/118. The devices and methods illustrated in FIG. 1can be modified to sequentially coat more than two coating compositionsonto a substrate. For example, additional coating dispensers, ovens, orlight sources can be added to the apparatus.

FIGS. 2 to 4 illustrate various stages of the method illustrated byFIG. 1. In one embodiment of the invention, as illustrated in FIG. 2,the first coating composition 302 is polymerized prior to application ofthe second coating composition. The polymerization optionally results inthe formation of a gradient of molecular weight along the thicknessdirection of the first coating composition. This polymerization can beinitiated by the activation of thermolabile free radical initiators orlight-sensitive free radical initiators (e.g., photoinitiators). Theaction of the free radicals can be inhibited by the presence of oxygen,or a suitable compound that can inhibit the action of the free radical,at the surface of the first coating composition. Heat or light forcuring can be supplied by the appropriate sources, for example, by anoven or a UV light, respectively. The duration or intensity, or both, ofthe heat or light can be used to control the extent of polymerization,which, in turn, can affect the gradient of molecular weight. The extentof polymerization can also be controlled by adjusting the relativeoxygen content present during the treatment.

As illustrated in FIG. 2, the first coating composition 302 is appliedto substrate 300. The first coating composition 302 includes, in asuitable solvent, a polymeric liquid crystal material or monomericliquid crystal material that can be cured prior to or after coating thesecond coating composition.

As illustrated in FIG. 3, a second coating composition 304 is applied tothe layer 303 formed using the first coating composition. When amolecular weight gradient is used, the molecular weight of the layer 303typically decreases from the substrate 300/layer 303 interface to thelayer 303/second coating composition 304 interface. The second coatingcomposition 304 includes at least one chiral monomer which can diffuseinto the layer 303. The rate and extent of the diffusion of the chiralcompound can be limited by the gradient of molecular weight.

As illustrated in FIG. 4, diffusion occurs from the second coatingcomposition 304 into layer 303 which is coated on the substrate 300 toform a region 306 in which the pitch of the cholesteric liquid crystalmaterial formed from the first and second compositions varies along athickness dimension. This region 306 can extend through only a portionof the original layer 303, as illustrated in FIG. 4, or through theentire layer 303.

In some embodiments in which the coating compositions include monomersthat are polymerized to form the cholesteric liquid crystal layer, themonomers can be partially polymerized before, during, or after coating,but before completing the diffusion between the two coatingcompositions. For example, one or more curing light or radiation sourcescan be positioned at one or more points along the processing path topartially cure (e.g., polymerize or crosslink) the monomers atparticular rates. This can be done to alter the diffusion rate andcontrol the pitch profile in the final cholesteric liquid crystallayer(s).

As an example, a broadband reflective polarizer can be formed accordingto the methods and configurations described herein. This broadbandreflective polarizer can substantially uniformly (e.g., with no morethan about 10% or 5% variation) reflect light of one polarization over awavelength range of 100 nm, 200 nm, or 300 nm or more. In particular, abroadband reflective polarizer can be formed that substantiallyuniformly reflects light of one polarization over the visible wavelengthrange (e.g., from about 400 to 750 nm).

DISPLAY EXAMPLES

The cholesteric liquid crystal optical bodies can be used in a varietyof optical displays and other applications, including transmissive(e.g., backlit), reflective, and transflective displays. For example,FIG. 5 illustrates a schematic cross-sectional view of one illustrativebacklit display system 400 including a display medium 402, a backlight404, a cholesteric liquid crystal reflective polarizer 408, as describedabove, and an optional reflector 406. The display system optionallyincludes a quarter wave plate as part of the cholesteric liquid crystalreflective polarizer or as a separate component to convert thecircularly polarized light from the liquid crystal reflective polarizerto linearly polarized light. A viewer is located on the side of thedisplay device 402 that is opposite from the backlight 404.

The display medium 402 displays information or images to the viewer bytransmitting light that is emitted from the backlight 404. One exampleof a display medium 402 is a liquid crystal display (LCD) that transmitsonly light of one polarization state.

The backlight 404 that supplies the light used to view the displaysystem 400 includes, for example, a light source 416 and a light guide418, although other backlighting systems can be used. Although the lightguide 418 depicted in FIG. 5 has a generally rectangular cross-section,backlights can use light guides with any suitable shape. For example,the light guide 418 can be wedge-shaped, channeled, a pseudo-wedgeguide, etc. The primary consideration is that the light guide 418 becapable of receiving light from the light source 416 and emitting thatlight. As a result, the light 418 can include back reflectors (e.g.,optional reflector 406), extraction mechanisms and other components toachieve the desired functions.

The reflective polarizer 408 is an optical film that includes at leastone cholesteric liquid crystal optical body, as described above. Thereflective polarizer 408 is provided to substantially transmit light ofone polarization state exiting the light guide 418 and substantiallyreflect light of a different polarization state exiting the light guide418.

FIG. 6 is a schematic illustration of one type of reflective liquidcrystal display 500. This reflective liquid crystal display 500 includesa display medium 502, a mirror 504, and a reflective polarizer 506. Thedisplay system optionally includes a quarter wave plate as part of thecholesteric liquid crystal reflective polarizer or as a separatecomponent to convert the circularly polarized light from the liquidcrystal reflective polarizer to linearly polarized light. Light 508 ispolarized by the reflective polarizer, travels through the displaymedium, bounces off the mirror, and goes back through the display mediumand reflective polarizer. The reflective polarizer of this reflectiveliquid crystal display 500 includes one cholesteric liquid crystaloptical body, as described above. The specific choice of cholestericliquid crystal optical body can depend on factors such as, for example,cost, size, thickness, materials, and wavelength range of interest.

The cholesteric liquid crystal optical body can be used with a varietyof other components and films that enhance or provide other propertiesto a liquid crystal display. Such components and films include, forexample, brightness enhancement films, retardation plates includingquarter-wave plates and films, multilayer or continuous/disperse phasereflective polarizers, metallized back reflectors, prismatic backreflectors, diffusely reflecting back reflectors, multilayer dielectricback reflectors, and holographic back reflectors.

Example 1

Different coating solutions were prepared for the coating procedure. Thecomposition of these coating solutions is listed in Table 1. Coatingsolution 4 is a mixture of solutions 1 and 2. Tetrahydrofuran (THF) andmethyl ethyl ketone (MEK) (both available from Aldrich Chemical Co.,Milwaukee, Wis.) were used as the solvents. The preparation of CompoundA is described in European Patent Application Publication No. 834754,which is incorporated herein by reference. The structure of Compound Ais:

Compound 756 (Paliocolor™ LC 756) and Compound 242 (Paliocolor™ LC 242)are liquid crystal monomers available from BASF Corp. (Ludwigshafen,Germany). Darocur™ 4265 (Ciba Geigy Corp., Basel, Switzerland) is aphotoinitiator. Vazo™ 52 (DuPont, Wilmington, Del.) is a thermallydecomposable substituted azonitrile compound used as a free radicalinitiator. The substrate used for coating had an alignment layer on itconsisting of stretched (by a factor of 6.8) polyvinyl alcohol (PVA)(Airvol 425, Air Products, Allentown, Pa.).

Coating solution 1 was prepared by dissolving the compounds of coatingsolution 1, as listed in Table 1, in THF at 60° C. Coating solution 1was then purged with nitrogen gas, sealed in a container, and heated at60° C. for 16 hours in order for polymerization of the liquid crystalmonomer to occur. Coating solutions 2 and 3 were prepared by dissolvingthe indicated compounds in the solvents at 60° C. Coating solution 4 wasprepared by mixing solutions 1 and 2, and then adding the photoinitiatorat room temperature.

The optical body was prepared by applying coating solution 4 on the PVAsubstrate using a #20 wire wrapped rod. Coating solution 4 was appliedto give a thickness, when dried, of approximately 7.5 micrometers. Thecoating was air dried for 5 minutes at room temperature and then placedinto a 11 0° C. oven for 10 minutes to align the polymer. Next, thecoating was UV cured in air using a 300 watt/in. Fusion™ conveyor UVcuring system (Fusion MC-6RQN; Fusion UV Systems, Inc., Gaithersburg,Md.) and a Fusion™ D lamp. The dose was approximately 1.5 J/cm². Thecoating was cured at 20 ft/min. using two passes.

Coating solution 3 was subsequently applied onto cured coating solution4 also using a #14 wire wrapped rod. The coating was again air dried 5minutes at room temperature. Coating solution 3 was applied to give athickness, when dried, of approximately 5 micrometers. The substratecontaining the two coatings was placed into a 90° C. oven for 15 min. toallow diffusion of the coating compositions to occur. The substratecontaining the two coatings was again UV cured in air using the 300watt/in. Fusion™ conveyor UV curing system. The dose was approximately1.5 J/cm². The coating was cured at 20 ft/min. using two passes.

Finally, a Lambda™ 900 spectrophotometer (Perkin Elmer, Santa Clara,Calif.) was used to measure the optical performance of the optical body.A quarter-wave film was placed in front of the coating and a standardlinear polarizer was placed in the light path and the transmissionthrough the coating was measured in a range from 400 nm to 700 nm. Thetransmission was measured with the linear polarizer rotated both +45°and −45° from the quarter-wave film to give parallel andcross-polarization results. The results of this transmission over themeasured wavelength range are indicated in FIG. 7.

Example 2

Coating solution 5 was prepared by dissolving the compounds of coatingsolution 5, as listed in Table 1, in THF at 60° C. Coating solution 5was then purged with nitrogen gas, sealed in a container, and heated at60° C. for 16 hours in order for polymerization of the liquid crystalmonomer to occur. Coating solutions 6 and 7 were prepared by dissolvingthe indicated compounds in the solvents at 60° C. Coating solution 10was prepared by mixing solutions 5 and 6, and then adding Lucirin™ TPO(BASF Corp., Ludwigshafen, Germany), at room temperature.

An optical body was prepared by applying coating solution 10 on the PVAsubstrate using a #26 wire wrapped rod. Coating solution 10 was appliedto give a thickness, when dried, of approximately 10 micrometers. Thecoating was air dried for 5 minutes at room temperature and then placedinto a 1 15° C. oven for 10 minutes to align the polymer. Next, thecoating was UV cured in air using a 300 watt/in. Fusion™ conveyor UVcuring system (Fusion MC-6RQN) and a Fusion™ H bulb. The dose wasapproximately 1.2 J/cm². The coating was cured at 20 ft/min. using threepasses from the backside of the film.

Coating solution 7 was subsequently applied onto cured coating solution10 using a #14 wire wrapped rod. The coating was again air dried 5minutes at room temperature. Coating solution 7 was applied to give athickness, when dried, of approximately 5 micrometers. The substratecontaining the two coatings was placed into a 105° C. oven for 6 min. toallow diffusion of the coating compositions to occur. The substratecontaining the two coatings was again UV cured in air using the 300watt/in. Fusion™ conveyor UV curing system and a Fusion D bulb under anitrogen atmosphere. The coating was cured at 20 ft/min. using twopasses.

Finally, a Lambda™ 900 spectrophotometer (Perkin Elmer, Santa Clara,Calif.) was used to measure the optical performance of the optical body.A quarter-wave film was placed in front of the coating and a standardlinear polarizer was placed in the light path and the transmissionthrough the coating was measured in a range from 400 nm to 700 nm. Thetransmission was measured with the linear polarizer rotated both +45°and −45° from the quarter-wave film to give parallel andcross-polarization results. The results of this transmission over themeasured wavelength range are indicated in FIG. 8.

Example 3

Coating solution 8 was prepared by dissolving the compounds of coatingsolution 8, as listed in Table 1, in THF at 60° C. Coating solution 8was then purged with nitrogen gas, sealed in a container, and heated at60° C. for 16 hours in order for polymerization of the liquid crystalmonomer to occur. Coating solutions 9 and 12 were prepared by dissolvingthe indicated compounds in the solvents at 60° C. Coating solution 11was prepared by mixing solutions 8 and 9, and then adding thephotoinitiator at room temperature.

The optical body was prepared by applying coating solution 11 on the PVAsubstrate using a #20 wire wrapped rod. Coating solution 11 was appliedto give a thickness, when dried, of approximately 7.5 micrometers. Thecoating was air dried for 5 minutes at room temperature and then placedinto a 120° C. oven for 10 minutes to align the polymer. Next, thecoating was UV cured in air using a 300 watt/in. Fusion™ conveyor UVcuring system (Fusion MC-6RQN) and a Fusion™ D lamp. The dose wasapproximately 1.5 J/cm². The coating was cured at 20 ft/min. using twopasses from the backside of the film.

Coating solution 12 was subsequently applied onto cured coating solutionII also using a #20 wire wrapped rod. The coating was again air dried 5minutes at room temperature. Coating solution 12 was applied to give athickness, when dried, of approximately 7.5 micrometers. The substratecontaining the two coatings was placed into a 80° C. oven for 10 min. toallow diffusion of the coating compositions to occur. The substratecontaining the two coatings was again UV cured using the 300 watt/in.Fusion™ conveyor UV curing system under a nitrogen atmosphere. The dosewas approximately 1.5J/cm². The coating was cured at 20 ft/min. usingtwo passes.

Finally, a Lambda™ 900 spectrophotometer (Perkin Elmer, Santa Clara,Calif.) was used to measure the optical performance of the optical body.A quarter-wave film was placed in front of the coating and a standardlinear polarizer was placed in the light path and the transmissionthrough the coating was measured in a range from 400 nm to 700 nm. Thetransmission was measured with the linear polarizer rotated both +45°and −45° from the quarter-wave film to give parallel andcross-polarization results. The results of this transmission over themeasured wavelength range are indicated in FIG. 9.

Example 4

Coating solution 13 was prepared by dissolving the compounds of coatingsolution 13, as listed in Table 1, in THF at 60° C. Coating solution 13was then purged with nitrogen gas, sealed in a container, and heated at60° C. for 16 hours in order for polymerization of the liquid crystalmonomer to occur. Coating solution 12 was prepared as indicated above.

The optical body was prepared by applying coating solution 13 on the PVAsubstrate using a #16 wire wrapped rod. Coating solution 13 was appliedto give a thickness, when dried, of approximately 6 micrometers. Thecoating was air dried for 5 minutes at room temperature and then placedinto a 130° C. oven for 10 minutes to align the polymer. Next, coatingsolution 12 was subsequently applied onto coating solution 13 also usinga #16 wire wrapped rod. The coating was again air dried 5 minutes atroom temperature. Coating solution 12 was applied to give a thickness,when dried, of approximately 6 micrometers. The substrate containing thetwo coatings was placed into a 90° C. oven for 7 min. to allow diffusionof the coating compositions to occur. The substrate containing the twocoatings was then UV cured using the 300 watt/in. Fusion conveyor UVcuring system under a nitrogen atmosphere. The dose was approximately1.5J/cm². The coating was cured at 20 ft/min. using two passes.

Finally, a Lambda™ 900 spectrophotometer (Perkin Elmer, Santa Clara,Calif.) was used to measure the optical performance of the optical body.A quarter-wave film was placed in front of the coating and a standardlinear polarizer was placed in the light path and the transmissionthrough the coating was measured in a range from 400 nm to 700 nm. Thetransmission was measured with the linear polarizer rotated both +45°and −45° from the quarter-wave film to give parallel andcross-polarization results. The results of this transmission over themeasured wavelength range are indicated in FIG. 10. TABLE 1 Solutions(Weight % based on Total Weight of Solution) 1 2 3 5 6 7 8 9 12 13Cmpd.A 14.25 14.25 — 14.3 14.3 — 14.35 14.35 — 14.4 LC756 0.75 0.75 1.50.7 0.7 1.8 0.675 0.675 1.05 0.6 LC242 — — 13.5 — — 13.2 — — 13.95 — THF85 85 — 85 85 — 85 85 — 85 MEK — — 85 — — 85 — — 85 — Dar.4265 — — 0.3 —— 45 — — 0.3 — Vazo 52 0.8 — — 0.6 — — 0.4 — — 0.4

TABLE 2 (Weight % based on Total Weight of Solutions from Table 1) 4 1011 Soln. 1  70% — — Soln. 2  30% — — Dar. 4265 0.6% — 0.30% Soln. 5 —  70% — Soln. 6 —   30% — Soln. 8 — —   70% Soln. 9 — —   30% TPO —0.45% —

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled) 6.(canceled)
 7. (canceled)
 8. (canceled)
 9. (canceled)
 10. (canceled) 11.(canceled)
 12. (canceled)
 13. A method of making an optical body, themethod comprising steps of: forming a first layer on a substrate using afirst coating composition; coating a second coating composition onto thefirst layer, the first and second coating compositions being differentand each of the first and second coating compositions comprising atleast one chiral material selected from chiral compounds, the secondcoating composition further comprising a solvent, wherein the firstlayer is substantially insoluble in the solvent of the second coatingcomposition; and diffusing at least a portion of the second coatingcomposition into a portion of the first layer adjacent the secondcoating composition; and forming at least one cholesteric liquid crystallayer from the first layer and second coating composition afterdiffusing the second coating composition into the first layer.
 14. Themethod of claim 13, wherein the chiral compounds of the chiral materialof the second coating composition are selected from cholesteric liquidcrystal compounds, and cholesteric liquid crystal monomers.
 15. Themethod of claim 13, wherein forming the first layer comprises coatingthe first coating composition onto the substrate, wherein the at leastone cholesteric liquid crystal material of the first coating compositionis substantially insoluble in the solvent of the second coatingcomposition.
 16. The method of claim 13, wherein forming the first layercomprises coating the first coating composition onto the substrate andcuring the first coating composition to form a polymeric liquid crystalmaterial, wherein the polymeric liquid crystal material is substantiallyinsoluble in the solvent of the second coating composition.
 17. Themethod of claim 13, wherein the second coating composition comprises achiral liquid crystal monomer material and an achiral material, whereindiffusing the second coating composition into the first layer comprisesdiffusing the chiral liquid crystal monomer material of the secondcoating composition into the first layer.
 18. The method of claim 17,wherein diffusing the chiral liquid crystal monomer comprises diffusingthe chiral liquid crystal monomer material into the first layer occursat a faster rate than diffusing the achiral material into the firstlayer.
 19. The method of claim 13, wherein the second coatingcomposition comprises a chiral monomer material and wherein diffusingthe second coating composition into the first layer comprises diffusingthe chiral monomer material of the second coating composition into thefirst layer.
 20. The method of claim 19, wherein the second coatingcomposition further comprises an achiral material and wherein the stepof diffusing, diffusion of the chiral material into the first layeroccurs at a faster rate than diffusion of the achiral material into thefirst layer.
 21. The method of claim 13, wherein the first and secondcoating compositions each comprise a liquid crystal compound having aspacer, wherein the spacers of the first and second coating compositionsdiffer to provide a different solubility to the respective liquidcrystal compounds.
 22. An optical body, comprising: a substrate; and acholesteric liquid crystal layer disposed on the substrate, thecholesteric liquid crystal layer having a non-uniform pitch along athickness direction of the cholesteric liquid crystal layer, thecholesteric liquid crystal layer comprising a crosslinked polymermaterial that substantially fixes the cholesteric liquid crystal layerto hinder diffusion of cholesteric liquid crystal material within thecholesteric liquid crystal layer.
 23. The optical body of claim 22,wherein the pitch of at least a portion of the cholesteric liquidcrystal layer changes monotonically along a thickness direction.
 24. Theoptical body of claim 22, wherein at least a portion of the cholestericliquid crystal layer adjacent the substrate has a uniform pitch.
 25. Theoptical body of claim 22, wherein a portion of the cholesteric liquidcrystal layer opposite the substrate has a pitch such that the portionreflects light having a wavelength outside of a range from 400 to 750nm.
 26. An optical display, comprising: a display medium; and areflective polarizer comprising a substrate; and a cholesteric liquidcrystal layer disposed on the substrate, the cholesteric liquid crystallayer having a non-uniform pitch along a thickness direction of thecholesteric liquid crystal layer, the cholesteric liquid crystal layercomprising a crosslinked polymer material that substantially fixes thecholesteric liquid crystal layer to hinder diffusion of cholestericliquid crystal material within the cholesteric liquid crystal layer.